1. Field of the Invention
This invention relates to adhesive and sealant compositions wherein one or more of the reactive and/or curable or polymerizable constituents of said compositions are encapsulated. It more particularly relates to encapsulated cure systems for, directly or indirectly, initiating and/or effectuating the cure or polymerization of adhesive and sealant compositions, as well as methods of making the same.
2. Description of Related Art
The use of microencapsulated ingredients in the manufacture of pharmaceuticals, pesticides, paints, adhesives, sealants and printing inks is well known. Perhaps the most widely known use of microcapsules has been in the product generally referred to as carbonless paper which incorporate or have applied thereto coatings comprising microencapsulated inks and/or other color forming agents that are released upon the application of pressure to the paper surface. In the manufacturing and service sectors one of the most widely known use of microcapsules has been in the manufacture of adhesives and sealants. Here the use of microencapsulation allows one to form a one-part adhesive or sealant from a typical two- or more-part adhesive and sealant system. Microencapsulation also allows one to pre-apply such adhesives and sealants to substrates at the site of manufacture or conversion rather than at the site of use or installation. The use of microencapsulation in adhesive and sealants is well known and takes many different paths.
Encapsulated solvent-based adhesive systems are of several different constructs. Roesch et al. (U.S. Pat. No. 5,922,798) teach solvent based adhesive compositions wherein a solvent, alone or together with a therein dissolved resin, is encapsulated and used to bond two substrates, each of which is dissolved or softened by the solvent. Eichel (U.S. Pat. No. 2,907,682) discloses adhesive tapes wherein the adhesive portion comprises a combination of encapsulated solvent and encapsulated solid adhesives, which are soluble in the solvent. Upon application of the tape to a substrate and the further application of pressure, the capsules rupture allowing the solvent to dissolve or at least tackify the adhesive whereby, upon evaporation or absorption of the solvent, an adhesive bond is formed. Where the solvent is non-evaporating, e.g., a plasticizer, and neither the substrate carrying the adhesive nor the substrate to which it is to be bonded absorb the non-volatile solvent, the result of the combination of the solvent and adhesive material is a pressure sensitive adhesive. Adhesives based on polyvinyl acetate, rubber, nitrile rubber, ethyl cellulose, or other cellulose derivatives such as cellulose acetate are especially suited for the solvent activation/reactivation type application.
Hot melt adhesive systems employing encapsulated solvents are also known. Baetzold et. al. (U.S. Pat. No. 6,084,010) teaches solid, tacky or non-tacky hot melt glue compositions having incorporated therein microcapsules of a solvent capable of softening or further tackifying the hot melt. Rubbing the hot melt composition, typically in the form of a stick, on the substrate to which it is to be applied fractures the capsules, releasing the solvent which, in turn, softens the hot melt and allows it to be deposited on the surface of the substrate.
Another type of encapsulated adhesive and sealant is that where the adhesive or sealant material or, in the case of a curable adhesive or sealant material, the components thereof are encapsulated in a single capsule. These capsules are typically applied to a substrate in a binder system that is non-tacky and dry to the touch. In this way, otherwise tacky or liquid flowable adhesives can be pre-applied, but not activated or bond forming until the capsule walls themselves are fractured releasing or exposing the adhesive materials. For example, Eichel (U.S. Pat. No. 2,986,477) teaches the encapsulation of tacky adhesive materials. Wallace (U.S. Pat. No. 4,428,982) teaches the encapsulation of curable anaerobic adhesives wherein the encapsulating material is air permeable so that the curable adhesive remains in a liquid or uncured state in the capsule until use. Schwantes (U.S. Pat. No. 6,592,990) teaches encapsulated adhesives, particularly pressure sensitive adhesives, wherein the adhesive is formed in-situ, after encapsulation of the ingredients therefore.
A third, and perhaps the most common use of encapsulation in adhesives and sealants, involves curable or reactive adhesive and sealant compositions which rely upon the presence of curatives or curing and/or cross-linking agents and/or other reactants such as activators, catalysts, initiators, accelerators, and the like for effecting polymerization or curing of the composition so as to form the desired adhesive or sealant, wherein one or more of the reactive constituents is encapsulated so as to isolate it from the other constituents. These adhesive and sealant compositions may be of a number of different types: some of the more typical being those based on epoxies, urethanes, unsaturated polyesters, alkyds, and (meth)acrylates, as monomers, pre-polymers, and low molecular weight polymers or combinations thereof. Such adhesive and sealant compositions are more typically found in the form of and characterized as two or more part systems wherein the parts are combined by the applicator at the time of use. However, with the advent of encapsulation technology, it is possible to encapsulate one or more of the reactive constituents, isolating it from the other constituents, so as to produce storage stable, one-part adhesive and sealant compositions. For example, the curatives or curing and/or cross-linking agents and/or other reactants may be encapsulated and said capsules dispersed in the liquid polymerizable monomer which forms the matrix of the adhesive or sealant. Alternatively, the liquid polymerizable component may be encapsulated and the curative or curing agent adhered to the outer wall of the capsule or the liquid polymerizable component may itself be encapsulated as well.
Oftentimes these one-part adhesive and sealant compositions encompass several different microcapsules, each containing one of the curatives or curing and/or cross-linking agents and/or other co-reactive constituents alone or together with other constituents of the adhesive or sealant composition. For example, with a free-radical polymerizable adhesive or sealant composition, so long as the oxidizing agent (typically the peroxide) and the reducing agent (typically an amine and/or metallocene) are in separate capsules, the system is stable. Here two different microcapsules may be employed wherein each contains a portion of the polymerizable component and one or more of the aforementioned curing agents and/or co-reactants. Similarly, for co-reactive polymerizable systems, each of the co-reactive species is encapsulated in separate microcapsules.
In the case of pre-applied adhesive and sealant compositions, typically a liquid adhesive or sealant composition containing the encapsulated component(s) dispersed therein is applied to the substrate and a polymer film formed over and encasing the liquid adhesive or sealant on the substrate to which it is applied. The polymer film holds the liquid adhesive or sealant composition in place and forms a protective barrier as well as a dry to the touch surface for the adhesive or sealant composition. Here, however, the curable or polymerizable component of the adhesive or sealant is not present in an encapsulated form, i.e., is not in the form of discrete microcapsules.
A second embodiment for the pre-applied adhesive and sealants is that where one or more of the components of the adhesive or sealant, including, in particular, the liquid polymerizable components, is encapsulated in a plurality of microcapsules which are dispersed in a liquid curable, polymerizable, or hardenable binder system. In this embodiment, one or more solid curative or curing agents or one or more solid activators, catalysts, initiators, accelerators, and the like for effecting curing or polymerization of the encapsulated liquid polymerizable component may be dispersed in the binder, without encapsulation. Alternatively, all of the components of the adhesive or sealant composition may be encapsulated in a plurality of different microcapsules which are dispersed in the liquid binder. In use, each of these modified binder systems is then applied to the intended substrate and allowed to cure, polymerize or harden; thereby binding the microcapsules to the substrate surface. Suitable binder systems may or may not co-react with the adhesive or sealant composition during cure or polymerization of the same. Most often the binders do not co-react, but instead act as a filler of the polymerizable or curable compositions.
Activation of these encapsulated adhesive and sealant compositions is effectuated by crushing the microcapsules so that the liquid polymerizable component comes into contact with the curative, curing agent, activator, catalyst, initiator, accelerator or the like. Mixing of the ingredients is reliant upon flow caused by the application of pressure and/or relative movement of the substrates to be bonded. Thus, because of the limited mixing, it is important that such systems be as fluid as possible to maximize the opportunity for mixing. Higher viscosity materials will result in less mixing and, thus, only partial curing of the curable materials. More importantly, higher viscosity materials are more difficult, if not impossible, to encapsulate. Where encapsulation is possible, particle size becomes an issue. Typically, in encapsulating high viscosity liquids, one tends to get a large number of large and small particles or microcapsules rather than a parabolic distribution where large and small particles are few.
While the advent of such encapsulated adhesive and sealant compositions has greatly broadened and/or simplified the end-use applications to which such adhesive and sealant systems are employed, they are not without remaining shortfalls. Premature fracturing of the capsules is perhaps the most prevalent of issues, especially in situations where the adhesive or sealant is used in rapid industrial manufacturing processes requiring quick delivery and application of the adhesive or sealant and, further, especially in the case of pre-applied adhesives or sealants, where the substrates to which the adhesives or sealants are pre-applied are likewise subject to such manufacturing processes, repeated handling or movement, etc. Concern for premature fracturing is not, however, limited to the application and use of the encapsulated adhesive. Such is also a great concern in the storage and handling of the microcapsules as well as the manufacture and processing of the adhesive or sealant itself, especially during incorporation of the encapsulated component into the matrix of the adhesive or sealant, or, in the case of pre-applied adhesive, incorporation of the encapsulated adhesive systems into the binder materials. Concerns with premature fracturing are heightened with systems that are thixotropic, have a high viscosity and/or incorporate fillers, especially granular fillers and fillers having sharp edges, or are subject to high shear mixing and dispensing processes.
Oftentimes to overcome concerns with premature fracturing, the thickness of the microcapsule walls is increased. This is especially found with pre-applied adhesives and sealants, particularly thread-locking or thread-sealing pre-applied adhesives and sealants, respectively. Consequently, cell walls comprising as much as 30 weight percent or more, more typically from 10% to 20% by weight, of the encapsulated component are not unusual in those one-part liquid systems to be applied at the time of use or from 15% to 25%, by weight in the case of pre-applied adhesives. However, as thicker and thicker walls are employed, less and less curable material is available with the same weight or volume of adhesive or sealant. Consequently, there is less curable material at the bond-line for forming the adhesive bond or seal. Because of the limited amount of curable material, there is poor flow and wetting of the substrate surfaces. Additionally, the large amount of shell wall fragments creates a gap between two substrates to be mated, much like grains of sand between plates of glass, which gap may be difficult to fill with the liquid curable material: again due to its limited quantity and low viscosity.
Furthermore, as the wall thickness increases, it becomes more and more difficult to break the cell walls when fracturing is desired. Thus, greater forces are needed to ensure the release of a comparable amount of the liquid curable or polymerizable component. Because rupture of the capsules in these instances is typically as a result of finger or hand pressure, pinch rollers and/or threading of threaded elements, the degree of rupture of the microcapsules lessens as the thickness of the capsule walls increases, especially if there is not a concurrent increase in the amount of pressure applied. This is particularly so for applications other than threading applications where multiple rotations are employed. Consequently, depending upon the specific end-use application, there may be poor or low sealant or bond strength due to the release of insufficient liquid curable or polymerizable components; thus, limiting performance or even the suitability of such materials for a given application. Similarly, as oftentimes found with the bonding and/or sealing of threaded elements, a larger volume of the pre-applied adhesive or sealant is necessary in order to attain the same bond or seal volume of cured material arising from the use of liquid, as opposed to pre-applied, adhesive and sealant compositions.
In order to moderate the need for thicker and thicker capsule walls, it is also known to incorporate hollow microspheres as “spacer” particles in the adhesive or sealant composition. For example, Hinterwaldner (U.S. Pat. No. 4,362,566) employs hollow microspheres to enhance storage stability of the adhesive formula as well as enhance fracturing of the microcapsules during activation and application of the adhesive material. However, the addition of such microspheres adds yet another additive to the system and, because the volume of the microspheres must be accommodated, requires the use of larger volumes of the adhesive and sealant to achieve the same bond or seal volume.
Consequently, it would be desirable to achieve encapsulated adhesive and sealant compositions in which the cell wall of the capsules is minimized and the quantity of liquid curable component is increased.
It would also be desirable to achieve encapsulated adhesive and sealant compositions having controlled flow characteristics, especially high or higher viscosities than are traditionally found with encapsulated adhesive and sealant compositions, and, in particular, having a fairly constant rheology over a larger temperature range, so as to enable their use in gap situations and other situations where squeeze-out or flow-out of the liquid curable material is undesirable.
It would also be desirable to achieve encapsulated adhesive and sealant compositions wherein, even if premature rupturing or fracturing of the capsules occurred, the composition was such that minimal, if any, reaction or polymerization of the curable or polymerizable components was able to occur.
It would also be desirable, especially for certain applications, to achieve the foregoing without the use of constituents or materials that are not relevant to the bonding and/or sealing of the substrates and surfaces to which they are to be applied.
It would also be desirable to achieve one-part storage stable adhesive and sealant compositions which are especially suited for use in high speed, industrial manufacturing processes.
It would also be desirable to achieve pre-applied adhesives which are able to withstand large pressures and forces without activation, especially in a pre-applied state on a given substrate.